Techniques have been developed to efficiently store information in a computer network. For example, the physical storage elements of a group of computer storage servers can be used to form a logical storage device, commonly referred to as a "virtual disk." The virtual disk is functionally equivalent to a single physical storage element but is actually formed of several physical storage elements.
For reasons analogous to the reasons why information stored at a physical storage element must be backed-up, the information stored at a virtual disk must also be backed-up. Because of the distributed nature of a virtual disk, however, special care must be taken to ensure that the same "version" of the virtual disk is copied from each of the physical storage elements that form the virtual disk.
To ensure consistency between the original and back-up copies of a virtual disk, prior to copying the virtual disk, the entire computer storage system is conventionally placed into a quiescent state. This basically causes the computer storage system to become inactive. The copy operation is permitted only after the computer storage system has reached quiescence. This procedure, although ensuring consistency between copies of a virtual disk, significantly intrudes on the normal operation of the computer storage system, including operations unrelated to the making of the copies. Accordingly, it would be advantageous to be able to copy virtual disks without intruding on normal computer storage system operations.